Hello, dear student!
While teaching the Russian language, my student and I have faced with the usage of the word "мыло" [mylo] (soap) in Russian. The point is that many foreigners have difficulty with its usage - how to say and write it correctly: "я люблю мыло" [ya l'yubl'yu mylo] or "я люблю мыла" [ya l'yubl'yu myla] (I like soap)? Many of them are confused and make mistakes. In fact, the word "мыло" is declined in Russian in singular and plural forms. Nevertheless, most specialists don't advise to use the plural form "мыла" [myla] in your speech. Let's analyze this case in detail:
Nominative case (что? - what?) мыло [mylo] - soap
Genitive case (чего? - what?) мыла [myla]
Dative case (чему? - what?) мылу [mylu]
Accusative case (что? - what?) мыло [mylo]
Instrumental case (чем? - what?) мылом [mylam]
Prepositional case (о чем? - about what?) о мыле [a myl'e]
We have declined the singular form of the word "мыло". As we can see from the example, actually, it's allright with the singukar form - it has the same endings as most Russian words. The plural form is a different story:
Nominative case (что? - what?) мыла [myla] - soaps
Genitive case (чего? - what?) мыл [myl]
Dative case (чему? - what?) мылам [mylam]
Accusative case (что? - what?) мыла [myla]
Instrumental case (чем? - what?) мылами [mylami]
Prepositional case (о чем? - about what?) о мылах [a mylakh]
We can notice in this example that the endings of plural form have changed and it's rather difficult to understand how to decline such nouns. I advise you just to memorize them. In my opinion, it's necessary to use the plural form of the word "мыло" [mylo] only in the cases when we want to emphasize the sort or variety of any soap. For example:
Я люблю французские мыла [Ya l'ubl'yu frantsuskie myla] - I love French soap
Я люблю мыться мылом [Ya l'ubl'yu mytsa mylam] - I like having a bath with soap
There are a lot of nouns in Russian which we use only in plural, e.g. "духи" [dukhi] - perfume, "сливки" [slifki] - cream, "консервы" [kans'ervy] - canned food, "хлопья" [khlop'ya] - flakes, "чернила" [chirnila] - ink. I have written about them in the previous article. However, when we want to say or point out the sort or variety of any product or thing, we use it in plural - here is also our word "мыло" / "мыла".
Несколько кусков мыла [N'eskal'ka kuskof myla] - Some pieces of soap
Интересные сорта мыла [Intir'esny'e sarta myla] - Interesting sorts of soap
The formation of the plural form is grammatically correct but most specialists do not advise to use it.